1. Field of the Invention
The present invention relates to electrical heating systems of the skin effect current type and, more particularly, to an improved heat generating system utilizing not only conduction but also convection in a two part cross sectional structure that is independent of the means being heated.
2. Description of Prior Art
The use of an alternating current generated power source for heating pipe lines conveying viscous fluids are well known, as seen in U.S. Pat. No. 2,802,520, wherein the heater element consisted of a co-axial arrangement of an insulated copper conductor and a copper pipe with one end of the conductor connected to one terminal of its alternating current source, the other end of the conductor connected to a far end of the copper pipe and the near end of the copper pipe connected to the other terminal of the alternating current power source. The heat generated by the heater was of a lower wattage output because of the lower resistivity of the copper pipe, as against the higher resistivity of a steel pipe.
In U.S. Pat. No. 3,293,407, the importance of using a highly resistive ferromagnetic pipe as the outer sheath of a co-axial arrangement with an electrically insulated copper conductor was taught for use in heating large diameter pipe lines through the skin effect procedure, both arcuate surfaces being joined by single line contact, that is, at their point of tangency. It was recognized from the teachings of U.S. Pat. No. 3,293,407 that a number of small diameter heat tubes would be required to heat large diameter pipe lines in order to take the fullest advantage of skin effect heating. In Japanese Pat. No. 629819, registered Jan. 5, 1972 (No. S46-19345, published May 31, 1971) the same inventor, Masao Ando, of U.S. Pat. No. 3,293,407, taught the use of a larger area of contact between the heat tube and the pipe line, thereby facilitating greater transfer of heat to the pipe line.
In U.S. Pat. No. 3,617,699 it is taught that by the use of a structural form other than a tube, such as a semi-round shaped element having its longitudinal edges continuously welded to the pipe line would function as a skin effect heating member with higher wattage output per foot, thereby reducing the number of heat tubes heretofore required. Another teaching employing the skin effect of alternating current is described in U.S. Pat. No. 3,706,872 wherein staggered stitch welding in lieu of continuous welding is employed for reducing the installation cost of that in U.S. Pat. No. 3,617,699. In both U.S. Pat. No. 3,617,699 and U.S. Pat. No. 3,706,872 patents a portion of the wall of the pipe being heated is utilized in conjunction with the semi-round structure of U.S. Pat. No. 3,617,699 and angular structure of U.S. Pat. No. 3,706,872 for the return leg of the alternating current. This is necessary in order to create the magnetic field necessary for skin effect heating. And in U.S. Pat. No. 3,718,804 the above prior art is reviewed indicating the fear of dielectric breakdown in such installations with resultant arcing and blow-through in the portion of the pipe wall being used as part of the return leg of the alternating current.
Skin effect heating elements that are currently in use are mostly of the tubular type which provide minimal contact with the surface to be heated, as is evident in U.S. Pat. No. 3,293,407 and in brochure entitled "Electric Heating System for Pipelines", Page 4, published by Pipe Heating Systems, Inc. of Brecksville, Ohio, and as indicated on page 5 of said brochure, the major portion of the heating element is encapsulated within the thermal insulation of the pipeline. Furthermore, the annular space within the tubular element forms a dead air space that also retards the flow of heat from the tubular element to the surface being heated. Publications illustrating insulation systems for large pipes and the like are described in brochure entitled "Insulation Systems, Metal-On," distributed by Johns-Manville, Ken-Caryl Ranch, Denver, Colorado and in brochure entitled "Poly-Therm Piping System" distributed by Perma-Pipe, a Division of Midwesco, Inc. of Niles, Illinois. Both brochures disclose several means of heat tracing and in the majority of cases the tracers are shown on the top side of the pipe being heated where the heat loss is greatest.